Hummer h3 leaking in rear passenger floorboard, how do i fix this?

Well Im assuming the car isn't sitting in water higher than the floor. So if rain water is leaking in, the first thing I would check is the weatherstrip arround the door. If that is out of place then I suppose a problem like yours could occur. Check the headliner for any signs of water exposure, if the water is coming from a hole in the roof or a missing bolt from the roof rail system it should show in the headliner. Other than that I can't specifically think of anything that would cause that. Good luck.

vance and hines